The present invention relates to the field of protective netting, for example, snow or ground netting. Purely by way of example, netting of this type may be used either alone as close-fitting netting laid on and fixed to the ground, or as part of protective barriers.
In mountainous regions, protective barriers are used widely to shelter and protect installations or infrastructures from possible landslides, falling rocks, or avalanches. These protective barriers are generally constituted by metal netting structures which are sufficiently flexible to absorb the energy of bodies such as stones, earth, avalanches or the like, which strike them and bear on them. The netting, which is supported by posts firmly fixed in the ground or to the rock, is generally formed by steel wires or cables crossed, preferably at right angles. At the intersection points the cables are clamped together by connection devices of various types, the most common of which comprise a pair of opposed, shaped plates between which the cables are clamped.
Connection devices of the known type indicated above have the disadvantage that they substantially increase the cost of the metal netting and hence of the barrier structure both because of the intrinsic cost of the materials and of the process for the manufacture of the plates, and because, in most cases, the cables have to be clamped manually at the intersection points. Since the protective barriers may be used to protect such large geographical areas that the barrier netting often has to cover considerable distances, it can be understood that the problem of reducing the cost of the netting is of primary importance in the field.
European patent 0428848 describes a system for producing barrier netting constituted by crossed metal cables. At each intersection, two cables are bound in the form of a cross by two wires each having its ends wound on a respective cable on opposite sides of the intersection point. This system is simple and inexpensive and can also be at least partially automated.
However, tests carried out by the Applicant have shown that the known system of the type indicated above is effective only when small loads are expected on the barrier netting. In fact, a joint between two crossed cables formed in accordance with the teachings of the above-mentioned patent was subjected to tensile tests in a direction perpendicular to the plane of the netting and to slip tests performed by exerting a pull on one of the two crossed cables in the plane of the netting. The results of the tests showed, first of all, a fairly high variability of the strength of the joint, probably because of difficulty in achieving uniform and balanced tightening of the windings of the wires on the cables. As a consequence of the wide variability of the results, it is impossible to guarantee that the strength of the protective netting as a whole, which comprises a very large number of such connections, will be greater than the lowest of the values obtained experimentally.
A second not very encouraging experimental result which emerged from the tests carried out consisted of the poor strength of the joint formed by the two wires which--although comparable with the average of the other known systems--is very far from the strength of the best (and most expensive) systems comprising clamping plates. The breakage of the joint brought about experimentally always took place by yielding of the wires, above all after a loosening of the joint had been noticed with consequent separation of the cables at the intersection. The use of larger-diameter wires did not solve the problem since the greater curvature to which the wires had to be subjected during the winding around the cables made it difficult to achieve optimal tightening and also caused work-hardening of the metal so that it became excessively brittle.